bpf: add TCP_SAVE_SYN/TCP_SAVED_SYN options for bpf_(set|get)sockopt

Adding support for two new bpf get/set sockopts: TCP_SAVE_SYN (set)
and TCP_SAVED_SYN (get). This would allow for bpf program to build
logic based on data from ingress SYN packet (e.g. doing tcp's tos/
tclass reflection (see sample prog)) and do it transparently from
userspace program point of view.

Signed-off-by: Nikita V. Shirokov <tehnerd@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/filter.c

@@ -4007,6 +4007,12 @@ BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
 					tp->snd_ssthresh = val;
 				}
 				break;
+			case TCP_SAVE_SYN:
+				if (val < 0 || val > 1)
+					ret = -EINVAL;
+				else
+					tp->save_syn = val;
+				break;
 			default:
 				ret = -EINVAL;
 			}
@@ -4032,21 +4038,32 @@ static const struct bpf_func_proto bpf_setsockopt_proto = {
 BPF_CALL_5(bpf_getsockopt, struct bpf_sock_ops_kern *, bpf_sock,
 	   int, level, int, optname, char *, optval, int, optlen)
 {
+	struct inet_connection_sock *icsk;
 	struct sock *sk = bpf_sock->sk;
+	struct tcp_sock *tp;
 
 	if (!sk_fullsock(sk))
 		goto err_clear;
-
 #ifdef CONFIG_INET
 	if (level == SOL_TCP && sk->sk_prot->getsockopt == tcp_getsockopt) {
-		if (optname == TCP_CONGESTION) {
-			struct inet_connection_sock *icsk = inet_csk(sk);
+		switch (optname) {
+		case TCP_CONGESTION:
+			icsk = inet_csk(sk);
 
 			if (!icsk->icsk_ca_ops || optlen <= 1)
 				goto err_clear;
 			strncpy(optval, icsk->icsk_ca_ops->name, optlen);
 			optval[optlen - 1] = 0;
-		} else {
+			break;
+		case TCP_SAVED_SYN:
+			tp = tcp_sk(sk);
+
+			if (optlen <= 0 || !tp->saved_syn ||
+			    optlen > tp->saved_syn[0])
+				goto err_clear;
+			memcpy(optval, tp->saved_syn + 1, optlen);
+			break;
+		default:
 			goto err_clear;
 		}
 	} else if (level == SOL_IP) {